Different parameters such as pH and microwave (MW) irradiation time (in MW-assisted synthesis methods) can affect the photo-physical properties of nano-materials via tuning the size of nanocrystals. In this research work, ZnSe semiconducting nanocrystals (NCs) were synthesized in aqueous medium at different pH. Growth of NCs was done employing microwave irradiation with the power of 1000 W at different irradiation times. According to the results of x-ray diffraction (XRD), no displacement was seen in the x-ray diffraction pattern peaks of ZnSe nanoparticles by altering the pH and microwave irradiation time. Generally, UV–Visible absorption spectra of the samples indicate that the increase of microwave irradiation time and the pH, resulted in the red shift of the absorption edge. The exact amount of the energy band gap of the nanoparticles was estimated by using the derivation of absorption spectrum fitting method (abbreviated as DASF) which was within the range of 3.488–3.721 eV depending on the pH and microwave irradiation time. Also, for the present NCs, using the values of energy gap, the optical charge carrier transition index (m) was evaluated, indicating crucially the direct gap nature. Depending to the microwave irradiation time, the size of the synthesized crystallites (obtained from XRD) were within the range of 1.74– 2.18 nm and no general certain trend was founded against pH; employing the precise data of energy gap, effective mass approximation gives a reasonable increasing trend of NCs size with increasing pH except for pH = 12.2 Other purposes of this study are gaining the additional photo-physical properties of Urbach energy (Etail), dielectric constant (e), and the refractive index (n).
